Formaldehyde is a toxic chemical substance that is commonly present in indoor and outdoor air pollution. Indoors, materials like furniture, carpets and household chemicals emit formaldehyde; outdoors, formaldehyde is generated through incomplete combustion of coal and fuels, and is commonly found in automotive emissions and stationary sources (stacks) that burn carbon-based fuels. There is considerable interest in having the ability to make accurate measurements of formaldehyde to assess the fate and health consequences of formaldehyde emissions, as well as to promulgate new regulations to control these emissions.
The current manufacturing process for calibration standards for formaldehyde makes use of an analytical-scale permeation device. This device generates formaldehyde vapor by heating alpha-polyoxymethylene (a solid polymer of formaldehyde) in a sealed vessel and allowing the small amount of generated formaldehyde monomer vapor to diffuse through a length of Teflon® PTFE tubing into a flowing gas stream. Although it does successfully produce formaldehyde gas for mixtures, the extremely low rate of formaldehyde emission makes this process very lengthy and impractical for large quantities of cylinders.
Although it is a small molecule with a low molecular weight, formaldehyde does not persist as a gas phase at high concentration in the pure form. Formaldehyde undergoes self-reaction to form polymers of itself (such as paraformaldehyde) and a variety of larger organic molecules by condensation reactions. Formaldehyde can be stabilized as a monomer in solutions with organic solvents or water (formalin).
However, aqueous solutions are not suitable for component additions to gas cylinders, as moisture almost always adversely affects mixture stability. Therefore, pure, dry and uncontaminated formaldehyde must be generated in-situ as needed from materials that emit formaldehyde when heated, such as paraformaldehyde, trioxane and even gum rubber tubing.
Describe herein is a new technique and a new apparatus for controlled generation of formaldehyde monomer vapor. This new process generates larger quantities and higher concentrations of formaldehyde, thereby facilitating faster production of mixtures in gas cylinders. This new process also minimizes decomposition of formaldehyde via self-reaction as well as formation of undesired side products.